This application claims the priority of Korean Patent Application No. 2003-89359, filed on Dec. 10, 2003, and No. 2004-89722, filed on Nov. 5, 2004 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference.
1. Field of the Invention
The present invention relates to a wavelength division multiplexing-passive optical network (WDM-PON)-based packet transceiver, and more particularly, to an apparatus that can improve the efficiency of up/downstream packet transceiving by reflecting a plurality of variables of packet transmission, such as a packet lengths, asymmetry of downstream and upstream traffics, and an instantaneous burst of a traffic amount, on a WDM-PON in which a light source is not used in an optical network terminal (ONT), so called a loop-back based WDM-PON.
2. Description of the Related Art
Since a loop-back based ONT (LB-ONT) in a WDM-PON is an ONT, which does not require a light source to which a wavelength is designated, there exist advantages in terms of manufacturing, management, and installation of products. However, since the ONT must use downstream light as upstream light by loop-backing the downstream light in the ONT, upstream information and downstream information must be distinguished.
Two methods for a loop-back based WDM-PON using downstream light as upstream light have been suggested.
First, there is an interleave method of arranging upstream information and downstream information not to be temporally overlapped. The interleave method is divided into a packet interleave method of arranging upstream packets and downstream packets to temporally cross and a bit interleave method of alternatively arranging upstream bits and downstream bits.
Second, there is a superposed modulation method of transmitting a downstream optical signal to an ONT by making the modulation depth of the downstream optical signal low and modulating the downstream optical signal with upstream information after making a modulated optical power uniform to some degree by amplifying the modulated optical power in a saturated-gain region. Or by making the modulation depth of the the upstream optical signal high and extracting upstream information in an OLT receiver regardless of the low modulated downstream optical signal by modulating upstream data using the downstream optical signal.
Problems of the methods described above are as follows.
1) Packet and bit interleave methods                Packet or bit interleave synchronization between an OLT and an ONT is necessary.        Can be difficultly applied to packets having variable lengths, and segmentation assembly and reassembly (SAR) for the packet interleave is required.        When asymmetry between a downstream and an upstream is greater, a waste of bandwidth is more increased.        
2) Superposed modulation method                Since a modulation depth of a downstream optical signal is low or since a modulation depth of an upstream optical signal is high, error occurrence possibility is high.        A modulation depth of a downstream optical signal must be adjusted according to a distance between an OLT and an ONT, and it is difficult to change a distance between subscribers.        In order to amplify an optical power modulated by an ONT in a saturated-gain region, expensive devices, such as a preamplifier, a semiconductor optical amplifier (SOA), and an external optical modulator, are necessary.        